1. Field of the Invention
The present invention pertains to a process for the separation of a fluorocarbon and hydrogen fluoride from a mixture of the fluorocarbon and hydrogen fluoride. More particularly, the invention relates to the separation of azeotropic mixtures of fluorocarbons from hydrogen fluoride. The most preferred fluorocarbon for this invention is 1,1,1,3,3- pentafluoropropane, which is also known as HFC-245fa.
2. Description of the Prior Art
It is well known to react hydrogen fluoride with various hydrochlorocarbon compounds in order to produce fluorocarbons (HFCs). Such fluorocarbons are useful as solvents, refrigerants, blowing agents and aerosol propellants, among other uses. Fluorocarbons are considered to be much more environmentally advantageous than hydrochlorocarbons since they are non-ozone depleting, non-flammable and non-toxic as compared to chlorine containing chlorocarbons. In the production of fluorocarbons, a typical product stream contains unreacted hydrogen fluoride, other starting reagents and by-products as well as the desired HFC. Various conventional separation techniques, for example distillation and aqueous scrubbing may separate certain by-products and starting materials from a product stream, however, particular difficulty can be experienced in removing fluorocarbons from hydrogen fluoride. This is especially true for those fluorocarbons having boiling points close to that of HF. One of these fluorocarbons is 1,1,1,3,3-pentafluoropropane, which is also known as HFC-245fa. HFC-245fa has a boiling point of about 14.degree. C. at standard atmospheric pressure, which makes it particularly useful as a blowing agent or aerosol propellant. HFC-245fa is itself well known in the art as described in U.S. Pat. No. 2,942,036, Canadian 684,687, EP 381 986A and JP 02,272,086. A method for its preparation is described in WO 95/04022. All of the foregoing patents are incorporated herein by reference. In a typical method of preparing HFC-245fa, precursor reagents are fluorinated with hydrogen fluoride. It would be desirable to produce substantially pure HFC-245fa, however, this has proved to be difficult since HFC-245fa and hydrogen fluoride form an azeotropic mixture which is substantially inseparable by distillation.
The prior art has suggested various methods of separating azeotropic mixtures of fluorocarbons. In this regard European patent application EP 0 472 391 suggests separating HFC-134a from a mixture with chlorine containing hydrochlorofluorocarbons using an extraction agent such as trichloroethylene or perchloroethylene, among others. European patent application EP 0 467 531 teaches a method of separating HFC-134a from a mixture of HFC-134a with HF by passing the mixture through a distillation column to separate the mixture form a residue of pure HFC-134a and then collecting the residue. U.S. Pat. No. 5,211,817 attempts a separation of fluorocarbons from azeotropic mixtures with HF by column distillation and withdrawing a vapor sidestream followed by introducing the sidestream into a rectifying column equipped with a condenser and operated at a high reflux ratio. These provide less than satisfactory solutions to the problem.
According to the present invention there is provided a method for separating a fluorocarbon and hydrogen fluoride from a mixture of the fluorocarbon and hydrogen fluoride by using sulfuric acid as an extracting agent. Sulfuric acid has been used heretofore to separate a gaseous mixture of HF from a chlorine containing chlorofluorocarbon, namely FC-22 as described in U.S. Pat. No. 3,873,629. However, sulfuric acid has not been known heretofore as an extracting agent for the separation of HF from non-chlorine containing fluorocarbons and hydrofluorocarbons.